The objective of the proposed study is to investigate in acute and chronic experiments the physiological, metabolic and neuropathological effects of photo-dynamic therapy (PDT) on nontumored cerebral tissue in the cat as a function of optical energy deposition. The experiment will document changes in cerebral blood flow and cerebral edema, in response to PDT. In vivo 1=H and 31-P nuclear magnetic resonance (NMR) spectroscopy will be employed to monitor cerebral metabolic energy status and tissue pH. In addition, cerebral tissue will be analyzed for neuropathological changes using light microscopy. The hypothesis to be tested is that nontumored cerebral tissue undergoes pathophysiological change after being subjected to PDT. These changes must be identified and documented so that effective PDT protocols may be developed for the treatment of cerebral neoplasm.